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4 An Introduction to SCADA Fundamentals and Implementation

An Introduction to SCADA
Fundamentals and
Supervisory Control And Data
Electric power generation,
transmission and distribution:
Electric utilities detect current flow and
line voltage, to monitor the operation of
circuit breakers, and to take sections of
the power grid online or offline.
Buildings, facilities and
environments: Facility
managers use SCADA to
control HVAC, refrigeration
units, lighting and entry
Manufacturing: manage parts
inventories for just-in-time
manufacturing, regulate
industrial automation and
robots, and monitor process
and quality control.
Mass transit: regulate electricity to
subways, trams and trolley buses; to
automate traffic signals for rail
systems; to track and locate trains
and buses; and to control railroad
crossing gates.
Water and sewage: State and
municipal water utilities use
SCADA to monitor and regulate
water flow, reservoir levels, pipe
pressure and other factors.
Traffic signals: regulates
traffic lights, controls traffic
flow and detects out-oforder signals.
SCADA control
Remote Telemetry Unit
Remote Telemetry Unit
16 Digital i/o
SITE monitoring
Time Sync
2 Analogue inputs
2 control outputs
1 RS232 port
10 RouteT LAN
Dial-up remote
Alarms from remote
A SCADA system performs four functions:
1. Data acquisition
2. Networked data communication
3. Data presentation
4. Control
These functions are performed by four kinds of SCADA components:
1. Sensors (either digital or analogue) and control relays that directly
interface with the managed system.
2. Remote telemetry units (RTUs). These are small computerized
units deployed in the field at specific sites and locations. RTUs
serve as local collection points for gathering reports from sensors
and delivering commands to control relays.
3. SCADA master units. These are larger computer consoles that
serve as the central processor for the SCADA system. Master
units provide a human interface to the system and automatically
regulate the managed system in response to sensor inputs.
4. The communications network that connects the SCADA master
unit to the RTUs in the field.
Data Acquisition
• SCADA system needs to monitor hundreds or thousands of sensors.
Sensors measure:
1. Inputs and outputs e.g. water flowing into a reservoir (input), valve
pressure as water is released from the reservoir (output).
2. Discrete inputs (or digital input) e.g. whether equipment is on or off, or
tripwire alarms, like a power failure at a critical facility.
3. Analogue inputs: where exact measurement is important e.g. to detect
continuous changes in a voltage or current input, to track fluid levels in
tanks, voltage levels in batteries, temperature and other factors that can
be measured in a continuous range of input.
• For most analogue factors, there is a normal range defined by a bottom
and top level e.g. temperature in a server room between 15 and 25
degrees Centigrade. If the temperature goes outside this range, it will
trigger a threshold alarm.
• In more advanced systems, there are four threshold alarms for analogue
sensors, defining Major Under, Minor Under, Minor Over and Major Over
Data Communication
A communications network is required to monitor multiple systems from a
central location.
•TREND: put SCADA data on Ethernet and IP over SONET.
• SECURITY: Keep data on closed LAN/WANs without exposing
sensitive data to the open Internet.
• Encode data in protocol format (use open, standard protocols and
protocol mediation)
• Sensors and control relays can’t generate or interpret protocol
communication - a remote telemetry unit (RTU) is needed to provide an
interface between the sensors and the SCADA network.
• RTU encodes sensor inputs into protocol format and forwards them to
the SCADA master;
• RTU receives control commands in protocol format from the master and
transmits electrical signals to the appropriate control relays.
Data Presentation
SCADA systems report to human operators over a master
station, HMI (Human-Machine Interface) or HCI (HumanComputer Interface).
SCADA master station has several different functions:
• continuously monitors all sensors and alerts the operator
when there is an “alarm”
• presents a comprehensive view of the entire managed
• presents more detail in response to user requests
•performs data processing on information gathered from
• maintains report logs and summarizes historical trends.
Selection of RTU’s
RTUs need to:
• communicate with all on-site equipment
• survive an industrial environment. Rugged construction and ability to withstand
extremes of temperature and humidity (it needs to be the most reliable element in your
• have sufficient capacity to support the equipment at a site (though should support
expected growth over a reasonable period of time).
• have a secure, redundant power supply for 24/7 working, support battery power and,
ideally, two power inputs.
• have redundant communication ports e.g. secondary serial port or internal modem to
keep the RTU online even if the LAN fails (multiple communication ports easily support a
LAN migration strategy)
• have nonvolatile memory (NVRAM) for storing software and/or firmware. New firmware
downloadable over LAN to keep RTU capabilities up to date without excessive site visits
• control local systems by themselves (Intelligent control) according to programmed
responses to sensor inputs
• have a real-time clock to accurately date/time stamp reports
• have a watchdog timer to ensure that the RTU restarts after a power failure.
Selection of SCADA Master
A SCADA master should display information in the most useful ways to human
operators and intelligently regulate managed systems. It should :
• have flexible, programmable soft controls to respond to sensor inputs
• allow programming for soft alarms (reports of complex events that track
combinations of sensor inputs and date/time statements).
• automatically page or email directly to repair technicians and provide detailed
information display in plain English, with a complete description of what activity
is happening and how to manage it.
• have tools to filter out nuisance alarms (to prevents operators from loosing
confidence and stop responding even to critical alarms)
• support multiple backup masters, in separate locations (primary SCADA
master fails, a second master on the network automatically takes over, with no
interruption of monitoring and control functions)
• support multiple open protocols to safeguard the SCADA system against
unplanned obsolescence.